The invention arose during continuing development efforts relating to ignition systems for marine engines in which ignition energy is discharged across a spark plug to provide enough spark to ignite a combustible mixture in a cylinder. The invention arose particularly in connection with efforts to improve production quality and operating life of paper wound ignition coils encapsulated in an electrically insulated housing made from a thermal setting polymer compound.
Paper wound ignition coils typically have a primary winding and a secondary winding impregnated in an electrically insulated (dielectric) housing made from an encapsulation compound which usually have solid additives such as sand fillers. Historically, strips of electrical grade paper such as kraft paper have been used to separate layers of wire windings in the primary and secondary windings. Investigation has found that the paper separators in the secondary winding tend to filter the sand fillers or other solid additives during the manufacturing process, thus creating filler dams in the housing and producing variations in encapsulating polymer composition. The variations in polymer composition result in dielectric constant discontinuities throughout the housing. Perhaps more importantly, these composition discontinuities result in a non-uniform shrink rate when curing the housing material during the manufacturing process and in non-uniform thermal expansion characteristics of the manufactured housing. These discrepancies in shrink rate and in the coefficient of thermal expansion cause the housing to crack or otherwise deteriorate, both during manufacturing and thereafter, thereby potentially reducing coil voltage output or, in the worst case, causing coil failure. Test observations also indicate that the absorption and/or adsorption characteristics of the paper wire separators alter the chemistry of the encapsulation compound within the winding, resulting in additional dielectric discontinuities.
The invention provides an ignition coil and a method of making the same which uses non-filtering/non-segregating secondary winding wire separators in lieu of paper separators. The preferred secondary winding separators are strips of nylon (solid filament) mesh having a mesh size of roughly 0.050 inches. Other types of non-filtering/non-segregating separators such as closed cell, non-segregating polyester cloth are also considered to be within the scope of the invention. Using a non-filtering secondary winding separator alleviates the problem of filler damming and renders the composition of the housing essentially homogeneous throughout the housing. Because the composition of the housing is essentially homogeneous, the dielectric, shrink rate and thermal expansion characteristics are generally the same throughout the housing, and many of the above-described problems are alleviated.
Another advantage of nylon, or other materials made of solid filaments, is that the nylon separator does not alter the chemistry of the encapsulation compound by absorption and/or adsorption.